The present invention relates to a valve for controlling the flow of the fluid through a bore and particularly, but not exclusively, the invention relates to a ball valve for use in the oil and chemical process industry.
Ball valves are commonly used in both industries. The type of ball valve of interest in relation to controlling flow of a fluid is an apertured ball valve such as is disclosed in applicant""s co-pending published Patent Application No. WO 93/03255 which was published on Feb. 18, 1993. In an apertured ball valve the valve operation or function may be broken down into two separate stages. Firstly, the ball moves between an open and a closed position by rotating through 90xc2x0 such that the ball aperture from an orientation coaxial with the flow direction, i.e. when the valve is open, to a position whereby the ball aperture is normal or perpendicular to the flow direction. Secondly, the valve seals in the closed position to prevent flow through the bore across the ball valve. Therefore, the on-off control of flow through the valve is achieved by rotating the ball through 90xc2x0 within the valve housing.
There are two basic types of ball valve mechanism which currently exist which fulfil the above functions. Firstly, there is the trunnion mounted ball system in which the ball element is positionally constrained inside the valve, usually by radial bearings. The ball is rotated by the application of torque to the trunnion. Sealing occurs as a result of the valve seat xe2x80x9cfloatingxe2x80x9d onto the ball element. The advantage of this system is that it provides highly reliable rotation between the valve open and the closed positions. The principal disadvantage of this system is that seal reliability is reduced because the sealing force only develops in proportion to the annular area of the valve seat. Thus, when trunnion mounted ball systems are used in high pressure wells and especially those in which the well fluid has a high proportion of particulate matter, being generally known as xe2x80x9caggressivexe2x80x9d wells, the pressure is such that the particulate matter can leak past seals between the ball and the valve seats and become jammed in all surfaces of the valve. This often results in the valve not achieving integrity of sealing. In such cases this type of ball valve is unable to operate properly in such conditions.
The second type of ball valve mechanism which effects the abovementioned function is known as the xe2x80x9cfloating ball systemxe2x80x9d. In this system the ball is not positionally constrained relative to the valve body. Rotation is caused by the application of force to a point which is offset from the ball centre which, in conjunction with the mating curvatures of the ball and seat, cause the ball to rotate. Sealing occurs as a result of the ball xe2x80x9cfloatingxe2x80x9d onto the valve seat. The advantage of this mechanism is that the reliability of the seal is increased because the sealing force develops in proportion to the circular area of the ball to seat contact. The disadvantage of this type of mechanism is that the rotational reliability is reduced as the friction factor between the ball and seat are considerably larger than that of trunnion mounted devices. With aggressive types of wells and particulate flows of the type described above, the reliability of this valve creates a problem in that the valve seizes between the open and the closed position giving rise to serious problems in both operational and safety terms.
An object of the present invention is to provide improved ball valve which obviates or mitigates at least one of the aforementioned disadvantages.
This is achieved by allowing a very slight movement of a ball valve retaining mechanism to allow that the ball element to be unloaded off the valve seat during rotation, but remain in contact with the valve seat so as to prevent debris ingress between the ball and the valve seat, and to instantaneously reload onto the valve seat upon the event of closure. This instantaneous and automatic redirection of the reaction load path at the occurrence of closure provides an effective seal against high pressure aggressive fluids to prevent fluid escaping beyond the valve components whilst, at the same time, allowing effective rotational movement of the valve to occur without providing rotational reliability.
This solution allows conflicting load paths through the valve to be resolved, namely through the trunnion during rotation and through the valve seat during sealing.
The slight movement of the ball retaining mechanism which is required may be variable depending on a number of factors but requires to be only very slight and in the preferred arrangement hereinafter described is of the order of 0.025xe2x80x3 (0.60 mm).
According to one aspect of the present invention, there is provided a ball valve structure comprising:
a valve housing having a wall defining a housing bore having a longitudinal bore axis,
an apertured ball element disposed within a ball cage, said ball element and said ball cage being disposed within said bore for rotation between a first position in which said ball element is oriented such that the aperture of the ball element is aligned with the bore, this position defining a valve open position, and a second position in which said ball element is rotated through approximately 90xc2x0 such that said ball element fully obstructs said bore, this position being defined as the closed position,
piston means disposed within said housing and coupled to said ball element such that movement of said piston means in the direction of the longitudinal axis of said bore causes said ball element to rotate between said open and said closed position,
valve seat means disposed downstream of said ball element between said ball element and a valve housing cap, said valve seat means being coupled to first resilient means for applying a first spring force for biasing said valve seat into contact with said ball element as it moves between said open and said closed positions,
compression spring means coupled to said piston means and responsive to movement of said piston means to move between a first position defining a first compressive spring force sufficient to maintain said ball element in said closed position and a second compressed position in which said piston is actuated to move that said ball element to said open position, said first compression spring means being retained in a direction substantially parallel to said longitudinal bore axis within said housing by a top plate means and by said lower plate means,
a generally tubular mandrel coupled to said ball cage and to said bottom plate means, such that said mandrel, said ball cage and ball element and said valve seat are constrained to be moved together,
second resilient means disposed between the bottom plate means and said valve housing for applying a second spring force to said mandrel assembly for biasing said mandrel assembly, said ball element and said ball cage towards said valve seat, said second spring force being selected to be less than the force of said compression spring means when said ball element is in said open and closed position but greater than the spring force of said first resilient means when said ball valve is in the closed position,
the arrangement being such that in response to an applied force said piston means is moveable to rotate said ball element to an open position and to compress said compression spring means to a compressed state in which a compressed spring force in said compressed state which is greater than the second spring force applied by said second resilient means, and the first resilient means applies said first spring force to said valve seat to bias said valve seat to remain in contact with said ball element in said open position and,
in the absence or removal of the force applied to said piston means, said compression spring means urges said piston means towards said housing cap such that such ball element is rotated by substantially 90xc2x0 to a just-closed position where said top plate means abuts said tubular mandrel to limit the decompression of said first compression spring means and, substantially instantaneously, said second resilient means urges said tubular mandrel, said ball cage and said ball element upwardly by a minimal amount relative to said piston means against said valve seat to create a valve closed intensifying condition to provide a relatively strong and effective initial seal between said valve seat and said ball element.
Preferably, said piston means is a tubular or annular piston having apertures in an interior wall thereof for receiving pins or trunnions coupled to said ball element such that rectilinear movement of said piston within said valve housing causes said ball element to rotate substantially 90xc2x0 between a fully open and a fully closed position.
Preferably also, said ball cage surrounds the ball element and provides a sealed unit as said ball element rotates to prevent passage of debris from the bore of said housing to the components of the ball valve.
Advantageously, said compression spring means is provided by a stack of radially, and circumferentially, spaced helical coil springs, each coil spring bring retained between said top plate means and said lower plate means, said top plate means being moveable with said springs in response to force applied from said piston.
Conveniently, a lower seal ring is coupled to said mandrel beneath said lower plate means.
The second resilient means is provided by a Belleville spring disposed between the base of the lower seal ring and the valve housing. Similarly, the first resilient means is provided by a Belleville spring, which is coupled between the valve seat and the housing cap, for biasing valve seat into contact with said ball element.
Conveniently, said valve housing cap has a port and conduit to the interior of said housing cap for receiving hydraulic fluid for actuating said piston to move downwardly against the spring force of helical coil springs.
Conveniently, eighty helical coil springs are disposed around the periphery of the valve housing to supply a spring force of about 50,000 lbs (225 Kilonewtons (Kn)) when the valve is located in the open position and to supply a spring force of approximately 30,000 lbs (135 Kn) when said valve is in a closed position. The lower Belleville spring provides an upward spring force of approximately 25,000 lbs (112 Kn) and the upper Belleville spring provides a downward force of approximately 10,000 lbs (45 Kn).
Conveniently, the distance moved by the assembly of the mandrel, ball cage and ball element and valve seat assembly between the just-closed position and the intensified position is about 0.025xe2x80x3 (0.60 mm).
Advantageously, the pin or trunnion coupled to said ball element for rotation of the ball element by said piston is disposed in windows or apertures in said piston with clearance to allow the pin to float axially upwards thereby allowing the valve components to reposition themselves in response to spring forces and hydrostatic end loads to the intensifying condition.
According to another aspect of the present invention, there is provided a method of minimising the ingress of debris between a ball valve element and valve seat as the ball valve is moved between an open and a closed position, the method comprising the steps of, compressing the first compression spring means, having a first uncompressed spring force, to a second position having a second compressed spring force greater than said first compressed spring force, and simultaneously rotating said apertured ball element to said open position,
biasing a valve seat into contact with said ball element as the ball element rotates from the open to the closed position, said biasing being achieved using a first resilient means having a resilient spring force,
providing a second resilient spring for applying an upward force to an assembly consisting of a mandrel means, a ball cage and ball element and said valve seat, said second resilient spring force being less than the second compressed spring force of said first compression spring means when said valve element is in said open position but being greater than said spring force of said first resilient means,
moving the first compression spring means upwardly in the absence or removal of an applied force to cause the ball element to rotate through 90xc2x0 and to a just-closed piston, retaining said valve seat in contact with said ball element by said first resilient means, and
providing an upward biasing force flowing through said mandrel assembly, said ball cage and said ball element so as to move the assembly upwards by a relatively small amount to provide an intensifying effect between said ball element.
In accordance with another aspect of the present invention, there is provided a ball valve for use with said ball valve structure, said improved ball valve comprising an apertured ball valve having pin means coupled thereto for rotating the ball valve between an open and a closed position in response to an applied force, ball cage for surrounding said ball element except in the region of a valve bore, said cage sealingly engaging said valve seat on one side of the ball and being adapted to be coupled to a mandrel on the other wide of the ball thereby providing a mechanical barrier between the movement of debris from the bore of said ball valve structure to working components of said ball valve structure.
Preferably, said cage is formed by two matching split shells which, when coupled together, form the cage surrounding said ball element.
Each shell has a circular window therein for receiving and engaging with the trunnions of the ball element via plain bearings.
Preferably, the lower end of the cage is threaded into the mandrel.
Conveniently, the trunnions engage with a cylindrical piston which is adapted to be move rectilinearly within a valve housing in response to application of hydraulic pressure, such that when pressure is applied, or removed, the piston moves in the direction of the longitudinal axis of the bore such that the ball element moves within said cage between an open and a closed position.
According to a further aspect of the present invention, there is provided a method of equalising pressure across the ball valve in response to an over pressure from above the ball valve, the method comprising the steps of:
exposing a portion of the main valve actuating means to bore pressure above said valve element, causing downward movement of said valve actuating means in response to said over pressure to rotate the ball partially open to allow pump through of fluid.
It will be understood that the valve seat spring maintains the valve seat in contact with the ball element throughout the pressure equalisation process.